Liquid phase oxidation



Patented May 8, 1951 LIQUID PHASE OXIDATION William S. Emerson andRobert A. Heimsch, Dayton, Ohio, assignors to Monsanto Chemical Company,St. Louis, Mo., a corporation of Dela,-

ware

No Drawing.

This invention relates to liquid phase oxidation of alkyl substitutedaromatic compounds in the presence of metallic catalysts. More,specifically the invention relates to a method of promoting thecatalytic oxidation by conducting it in the presence of certainsubstances which provide free radicals.

The liquid phase oxidation of alkylbenzenes, for example xylene, in thepresence of metal oxide or metal salt catalysts is well known to theart. These oxidations have been quite useful in the preparation ofpartially oxidized prod- .ucts, for example toluic acid, but only verysmall proportions of the dibasic acids have heretofore been prepared,especially when p-dialkylbenzenes are oxidized.

The primary purpose of this invention is to provide an improved methodwhereby substantial yields of acids may be prepared by the oxidation ofalkylbenzenes. invention is to provide a new and more practicable methodof preparing both monobasic and dibasic acids by the oxidation ofdialkylbenzenes with gaseous oxygen. A still further purpose is toincrease the yield of terephthalic acid in the oxidation of p-xylene.

Liquid phase oxidations of alkylbenzenes are conventionally conducted inclosed retorts under pressure, if necessary, to confine the hydrocarbonat the temperature of reaction. Pressures from 50 to 1000 pounds persquare inch are useful and temperatures of 100 to 250 C. have been used.

and most of the oxides are probably converted to salts of the alkylcarboyxlic acids, for example cobalt toluate, and the catalytic efiectis achieved by the metal in this form. The oxidations are usuallyconducted by means of oxygen, air, oxygen enriched air, or any othermixture of oxygen and inert gas, in corrosion resistant reactors; forexample nickel, stainless steel or glass-lined steel. When oxygen isused in high concentrations there is some explosion hazard; andtherefore preferred use involves mixtures of oxygen and inert gaseswherein the oxygen is less than fifty percent by volume.

The oxidation of alkylbenzenes can be practiced with any compound havinga benzene nu- A further purpose of this a Application March 22, 1 949,Serial No. 82,924

' 8Claims. (01.260-524) cleus and at least one alkyl substituent, forexample toluene, o-xylene, m-xylene, p-xylene, mixed xylenes,ethylbenzene, the various isometric diethylbenzenes, and thehydrocarbons having one or more alkyl substituents, such as n-propyl,isopropyl and the different butyl radicals, as for example in cumene andcymene. Of particular importance are the dialkylbenzenes wherein thealkyl radicals may be the same or different and each radical has fromone to four carbon atoms.

It has been found that the chemical action is promoted by the presenceof certain substances which yield free radicals under conditions of thereaction. Not all substances known to be productive of free radicals areeffective in inducing the promotion effect, but the criticalcharacteristics 'of promoters have not yet been definitely ascertained.Accordingly, predictions of the operativeness of various free radicalsubstances are not possible. In accordance with this invention it hasbeen found that substantial increases in yield of both monobasic anddibasic. acids are obtained when the. reaction is conducted in thepresence of a small. proportion of hexachlorethane. In the practice ofthis invention it has been found desirable to use from 0.05 to 2.0percent by weight of hexachlorethane, based upon the weight of thealkylbenzenes charged.

It has been found that the reaction is autocatalytic and that sometimesa period of induction is required before rapid oxidation begins. Theinduction period is especially noticeable when a new reactor is used, orwhen an old reactor is used after not being used for a period of time orafter use for a difi'erent reaction. It is believed that traces ofproduct on thesurface of previously used reactors catalyzes the reactionand eliminates, or minimizes, the usual period of induction. Theaddition of a small quantity of an oxidation product, for examplemonocarboxylic acid or a dicarboxylic acid corresponding to thealkylbenzene being oxidized, is beneficial.

'In the oxidation of p-xylene to terephthalic acid a small quantity ofp-toluic acid may be used to reduce the length of the induction period.

Further details of the practice of this invention are set forth withrespect to the following specific examples.

Example 1 A stainless-steel autoclave was provided with a mechanicalagitator consisting of a three blade propeller type and an electricmotor suitable for driving the propeller at 1,750 revolutions perminute. A gas inlet tube was so positioned so as to be immersed in thereaction mass at a point just above the propeller. The autoclave wasvented through a water cooled condenser and then through a valve toregulate the flow of exit gas. The condenser was provided with a meansfor separating the unreacted xylene and returning it to the autoclave.The temperature was maintained automatically by means of an electricheater and a thermocouple. The flow of gas through the autoclave wasmaintained by means of an air stream at a constant pressure andregulated by means of the valve in the vent line.

The autoclave was charged with 500 grams of p-xylene, 1.5 grams ofcobaltous hydrate and 2.5 grams of p-toluic acid. The temperature wasmaintained at 160 to 170 C. for four hours, during which time thereactor was maintained at 200 pounds per square inch pressure, and airwas provided at 15 cubic feet per hour. The analysis of the productshowed a conversion of 10.3 percent to terephthalic acid.

Example 2 The procedure of the preceding example was duplicated, exceptthat the autoclave was also charged with 1.0 gram of hexachlorethane.The yield of terephthalic acid was increased to 18.9 percent.

Example 3 Each of the two preceding examples were cluplicated exceptthat m-xylene was used in place of p-xylene. Without the use ofhexachlorethane a yield of 4.2 percent of isophthalic acid was ob--tained; with the promoter a yield of 7.2 percent was obtained.

In copending application serial No. 82,921, filed March 22, 1949 byWilliam S. Emerson and Robert A. Heimsch, there are described andclaimed methods of conducting the liquid phase oxidation in two stagesof varying conditions of operation. The first stage of the reaction isconducted at lower temperatures and the second stage, after substantialoxidation has taken place, is conducted at higher temperatures. Ifdesired the initial stage may be conducted with a substantially loweroxygen supply which is increased as the reaction approaches completion.These expedients may be used in addition to the hexachlorethane andfurther improvements in efficiency thereby obtained.

The invention is defined by the following claims.

We claim:

1. The catalytic liquid phase oxidation of dialkylbenzene todicarboxylic acid, which comprises contacting a dialkylbenzene havingfrom one to four carbon atoms in the alkyl groups, with an oxygencontaining gas in the presence of an oxidation catalyst of the groupconsisting of metal oxides, metal salts, and mixtures thereof andhexachlorethane, at a temperature in excess of 140 C.

2. The catalytic liquid phase oxidation of dialkylbenzene todicarboxylic acid, which comprises contacting a dialkylbenzene havingfrom one to four carbon atoms in the alkyl groups, with an oxygencontaining gas in the presence of an oxidation catalyst of the groupconsisting of metal oxides, metal salts, and mixtures thereof andhexachlorethane.

3. The catalytic liquid phase oxidation of dialkylbenzene todicarboxylic acid, which comprises contacting a dialkylbenzene havingfrom one to four carbon atoms in the alkyl groups, with an oxygencontaining gas in the presence of an oxidation catalyst of the groupconsisting of metal oxides, metal salts, and mixtures thereof andhexachlorethane, at a temperature between 140 C. and 200 C.

4. The catalytic liquid phase oxidation of a xylene to a dicarboxylicacid, which comprises heating a xylene with an oxygen containing gas inthe presence of a cobalt containing catalyst and from 0.05 to 2.0percent by weight of hexachlorethane, at a temperature between 140 C.and 200 C.

5. The catalytic liquid phase oxidation of a xylene to a dicarboxylicacid, which comprises heating a xylene with an oxygen containing gas inthe presence of an oxidation catalyst and from 0.05 to 2.0 percent byweight of hexachlorethane, at a temperature between 140 and 200 C.

6. A catalytic liquid phase oxidation of a xylene to a dicarboxylicacid, which comprises heating a xylene with an oxygen containing gas inthe presence of a cobalt containing catalyst and hexachlorethane, at atemperature between 140 and 200 C.

7. The catalytic liquid phase oxidation of a xylene to a dicarboxylicacid, which comprises heating a xylene with an oxygen containing gas inthe presence of a cobalt containing catalyst and from 0.05 to 2.0percent by weight of hexa chlorethane, at a temperature above .C.

8. The catalytic liquid phase oxidation of pxylene to terephthalic acid,which comprises contacting p-xylene with air at a temperature of 140 to200 C., in the presence of from 0.05 to 2.0 percent by Weight ofhexachlorethane and a cobalt containing catalyst.

WILLIAM S. EMERSON. ROBERT A. HEIMSCH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,120,672 Mares June 14, 19382,245,528 Loder June 10, 1941 2,276,774 Henxe et a1. Mar. 17, 19422,391,740 Raley et al Dec. 25, 1945 2,479,067 Gresham Aug. 16, 1949

1. THE CATALYTIC LIQUID PHASE OXIDATION OF DIALKYLBENZENE TODICARBOXYLIC ACID, WHICH COMPRISES CONTACTING A DIALKYLBENZENE HAVINGFROM ONE TO FOUR CARBON ATOMS IN THE ALKYL GROUPS, WITH AN OXYGENCONTAINING GAS IN THE PRESENCE OF AN OXIDATION CATALYST OF THE GROUPCONSISTING OF METAL OXIDES, METAL SALTS, AND MIXTURES THEREOF ANDHEXACHLORETHANE, AT A TEMPERATURE IN EXCESS OF 140* C.